The present invention relates to the field of flexible wiring boards, particularly the field of multilayer flexible wiring boards.
Double-sided flexible wiring boards having patterned metal films on both sides of a resin film are widely used because of the high degree of freedom of interconnection.
The metal films on both sides of the resin film are electrically connected to each other. Conventional methods for connecting these metal films are explained below.
First, the through hole method is explained. Referring to FIG. 7(a), the reference number 110 represents a base material for flexible wiring boards having metal films 112, 113 consisting of a copper foil adhered to the top surface and the bottom surface of a polyimide film 111.
This base material 110 is punched with a drill or the like to form a through hole 118 as shown in FIG. 7(b). Then, the assembly is carbonized and then electroplated, so that a copper plating layer 115 grows within the through hole 118 and on the surfaces of the metal films 112, 113 to connect the two metal films 112, 113 via the copper plating layer 115 within the through hole 118, as shown in FIG. 7(c).
Secondly, the via hole method is explained. Referring to FIG. 8(a), a base material 120 having a polyimide film 121 adhered on a metal film 122 consisting of a copper foil is prepared and an opening 128 is formed in the polyimide film 121 by photolithography (FIG. 8(b)).
Thus, the metal film 122 is exposed at the bottom of the opening 128, and a copper thin film is formed by sputtering on the surface of the metal film 122 exposed at the bottom of the opening 128 and on the surface of the polyimide film 121 in this state followed by electroplating to form a copper plating layer 123 on the top surface of the polyimide film 121 and the inner face of the opening 128 and the top of the metal film 122 exposed at the bottom of the opening 128. This copper plating layer 123 is connected to the metal film 122 at the bottom of the opening 128.
However, the through hole connection method requires a long time to form many through holes 118 because through holes 118 are drilled one by one. Moreover, it cannot satisfy the demand for fine patterns because the size of through holes 118 is limited to about 0.2 mmxcfx86.
The metal film 123 formed by the via hole method is easily separated because of the weak adhesive power between the copper plating layer 123 and the polyimide film 121. Furthermore, defects such as pinholes are more liable to occur in the copper plating layer 123 to make inferior the reliability.
An object of the present invention is to overcome the disadvantages of the prior art described above and to provide a technique that can connect metal films without forming any opening.
In order to attain the above object, the present invention provides a process for manufacturing a flexible wiring board, comprising the steps of forming an uncured first resin film including a solvent on a first metal film, pressing bumps on a second metal film against said first resin film to force said bumps into said first resin film until the tops of said bumps come into contact with said first metal film, then patterning at least one of said first or second metal films, and heat-treating said first resin film while the top surface of said first resin film is at least partially exposed to cure said first resin film.
In one embodiment of the process for manufacturing a flexible wiring board according to the present invention, said uncured first resin film is semicured by heating it before said bumps are pressed against said first resin film.
In another embodiment of the process for manufacturing a flexible wiring board according to the present invention, said semicuring step takes place at a temperature lower than the boiling point of said solvent included in said uncured first resin film.
In another embodiment of the process for manufacturing a flexible wiring board according to the present invention, said semicuring step takes place at a temperature ranged from 80xc2x0 C. to 300xc2x0 C.
In another embodiment of the process for manufacturing a flexible wiring board according to the present invention, said first resin film is softened by heating it when said bumps are forced into said first resin film.
In another embodiment of the process for manufacturing a flexible wiring board according to the present invention said curing step is followed by applying ultrasonic wave to either one or both of said bumps and said first metal film to connect said bumps to said first metal film.
In another embodiment of the process for manufacturing a flexible wiring board according to the present invention, said step of curing said first resin film is preceded by patterning either one of said first or second metal film and ultrasonic treating the unpatterned metal film and then patterning it.
Another embodiment of the process for manufacturing a flexible wiring board according to the present invention further comprises the steps of forming a second resin film on the top surface of said patterned first or second metal film, then pressing bumps on a third metal film against said second resin film to force said bumps into said second resin film until they come into contact with said first or second metal film, then patterning said third metal film and then curing said second resin film.
In this embodiment, said uncured first resin film may be semicured by heating it before said bumps are pressed against said first resin film.
Another embodiment of the process for manufacturing a flexible wiring board according to the present invention further comprises the steps of forming a second resin film on the top surface of said patterned first or second metal film, then pressing bumps on a third metal film against said second resin film to force said bumps into said second resin film until they come into contact with said first or second metal film, then patterning said third metal film, then curing said second resin film and then applying ultrasonic wave to said bumps on said third metal film to connect said bumps to said first or second metal film.
In this embodiment, said curing step may be followed by applying ultrasonic wave to said bumps on said third metal film to connect said bumps to said first or second metal film. It is possible to apply ultrasonic wave indirectly to said bumps by applying ultrasonic wave to said first or second metal film to connect said bumps to said first or second metal film. It is also possible to apply ultrasonic wave to both said bumps and said first or second metal film.
In this embodiment, said uncured first resin film may also be semicured by heating it before said bumps are pressed against said first resin film.
The present invention also provides a flexible wiring board comprising a plurality of patterned metal films with a resin film being interposed therebetween among which adjacent two metal films are electrically connected to each other via bumps, wherein said resin film is cured after said bumps are pressed against the top surface of said resin film and forced into said resin film to electrically connect said two metal films via said bumps.
In one embodiment of the flexible wiring board according to the present invention, said resin film is cured by heat-treating it while the surface of said resin film is at least partially exposed between said patterned metal films.
In another embodiment of the flexible wiring board according to the present invention, one of said two adjacent metal films connected via said bumps is ultrasonically bonded to said, bumps.
According to the present invention as defined above, bumps are pressed against a first resin film and forced into the first resin film. Thus, the bumps can be contacted with the metal film underlying the first resin film without forming any opening in the first resin film. The first resin film is preferably softened by heating it when the bumps are forced into the first resin film.
The bumps may be forced into the resin film by applying ultrasonic wave to the bumps digging or softening the semicured resin film in contact with the bumps.
When the bumps are embedded into the first resin heated, the first and second metal films are adhered to the first resin film. When at least one of the first and second metal films is patterned in this state to form an opening, the top of the first resin film is exposed at the bottom of the opening.
In this case, the top surface of the first resin film is partially covered with the first or second metal film and partially exposed. When the first resin film is heated in this state, the solvent and moisture included in the first resin film or the moisture generated during the progress of the chemical reaction caused by heating is discharged from the exposed first resin film so that the first resin film is cured. This curing step gives a double-sided flexible wiring board.
The curing step allows the first resin film to thermally shrink and the bumps to be strongly pressed against the first metal film, whereby the first and second metal films are electrically connected via the bumps.
In this case, ultrasonic wave may be applied to cause ultrasonic vibration interface between the bumps and the first metal film after the first resin film has been cured, so that the first metal film and the bumps are bonded by ultrasonic vibration energy. Ultrasonic wave may be applied on either side of the first metal film or the second metal film.
The height of the bumps used for connecting metal films is preferably greater than the thickness of the first resin film in which the bumps are to be embedded.